They are like the copper bracelets that were supposed to be good for arthritis (John Wayne may be seen wearing one in many of his later movies). It's a fad and will be found not to be effective - unless from placebo effect.

Sure magnets are medically indicated, just not usually for humans. Ever hear of a cow magnet? Now, if you look like a cow, eat like a cow, and act like a cow, then maybe one of those magnets will help you.

The MRI machine is in a separate room as it is very sensitive.
It will pickup noise from the computer and other electronic equipment.
The magnet is rather strong, as the force on a metal part will become about 10 times stronger as the distance is divided by 2.
There will happen sometimes accedents with MRI systems, like a cleaning machine that is drawn into the magnet.